This is, what, the fourth or fifth time you have posted this question since March? I think the problem is that we are confused about what you are asking about. The double helix is made from two "bands" the spiral around each other. There really is no enclosed volume. The area, I presume, is the cross-section, which is no more than a circle, so .

That's probably the best answer you are going to get for this unless you can tell us why this is not a sufficient answer.